I can't e-file this year so I have to mail my return and pay with checks too. I did everything in the app and printed out everything so now I just need to know how to ship these papers. The instructions say the deadline is on April 17th so would that mean I can mail them anytime before then? Do I need any special stamps or can I just print out postage and go with priority mail flat rate envelope?
As long as your envelope is post marked before or on April 17th you will be fine and it will not be counted late. And yes you can use a regular stamp and envelope to mail your return to the IRS.
Yes, you can mail anytime before April 17th, but I would highly suggest you mail your return via certified mail. Anytime you mail any documents to the IRS (or any federal agency), you want to have an official record of your mailing and to ensure that it was received and acknowledged - its well worth the extra money if there was ever an issue with the receipt of your documents (especially when you owe money as the penalties for not filing and paying can apply).